Femoral and Tibial Bases

ABSTRACT

Various embodiments are directed to femoral and tibial bases that form structures of an implantable mechanical energy absorbing system. According to one embodiment, the bases include a low-profile body having a elongate and a curved body portion. One end of the base is elevated as compared to another end. An inner surface of the low-profile body has a raised portion extending along the elongate, straight portion of the low-profile body. The bases also include a plurality of openings positioned along the low-profile body for alignment and purposes of affixation to body anatomy.

BACKGROUND

Various embodiments disclosed herein are directed to structure forattachment to body anatomy, and more particularly, towards approachesfor providing mounting members for trans-articular implantablemechanical energy absorbing systems.

Joint replacement is one of the most common and successful operations inmodern orthopaedic surgery. It consists of replacing painful, arthritic,worn or diseased parts of a joint with artificial surfaces shaped insuch a way as to allow joint movement. Osteoarthritis is a commondiagnosis leading to joint replacement. Such joint replacementprocedures are a last resort treatment as they are highly invasive andrequire substantial periods of recovery. Total joint replacement, alsoknown as total joint arthroplasty, is a procedure in which all articularsurfaces at a joint are replaced. This contrasts with hemiarthroplasty(half arthroplasty) in which only one bone's articular surface at ajoint is replaced and unincompartmental arthroplasty in which thearticular surfaces of only one of multiple compartments at a joint (suchas the surfaces of the thigh and shin bones on just the inner side orjust the outer side at the knee) are replaced.

Arthroplasty, as a general term, is an orthopaedic procedure whichsurgically alters the natural joint in some way. Arthroplasty includesprocedures in which the arthritic or dysfunctional joint surface isreplaced with something else as well as procedures which are undertakento reshape or realigning the joint by osteotomy or some other procedure.A previously popular form of arthroplasty was interpositionalarthroplasty in which the joint was surgically altered by insertion ofsome other tissue like skin, muscle or tendon within the articular spaceto keep inflammatory surfaces apart. Another less popular arthroplastyis excisional arthroplasty in which articular surfaces are removedleaving scar tissue to fill in the gap. Among other types ofarthroplasty are resection(al) arthroplasty, resurfacing arthroplasty,mold arthroplasty, cup arthroplasty, silicone replacement arthroplasty,and osteotomy to affect joint alignment or restore or modify jointcongruity.

The most common arthroplasty procedures including joint replacement,osteotomy procedures and other procedures in which the joint surfacesare modified are highly invasive procedures and are characterized byrelatively long recovery times. When it is successful, arthroplastyresults in new joint surfaces which serve the same function in the jointas did the surfaces that were removed. Any chodrocytes (cells thatcontrol the creation and maintenance of articular joint surfaces),however, are either removed as part of the arthroplasty, or left tocontend with the resulting new joint anatomy and injury. Because ofthis, none of these currently available therapies arechondro-protective.

A widely-applied type of osteotomy is one in which bones beside thejoint are surgically cut and realigned to improve alignment in thejoint. A misalignment due to injury or disease in a joint related to thedirection of load can result in an imbalance of forces and pain in theaffected joint. The goal of osteotomy is to surgically re-align thebones at a joint such as by cutting and reattaching part of one of thebones to change the joint alignment. This realignment relieves pain byequalizing forces across the joint. This can also increase the lifespanof the joint. The surgical realignment of the knee joint by high tibialosteotomy (HTO) (the surgical re-alignment of the upper end of the shinbone (tibia) to address knee malalignment) is an osteotomy proceduredone to address osteoarthritis in the knee. When successful, HTO resultsin a decrease in pain and improved function. However, HTO does notaddress ligamentous instability—only mechanical alignment. Good earlyresults associated with HTO often deteriorate over time.

Other approaches to treating osteoarthritis involve an analysis of loadswhich exist at a joint and attempts to correct (generally reduce) theseloads. Both cartilage and bone are living tissues that respond and adaptto the loads they experience. Within a nominal range of loading, boneand cartilage remain healthy and viable. If the load falls below thenominal range for extended periods of time, bone and cartilage canbecome softer and weaker (atrophy). If the load rises above the nominallevel for extended periods of time, bone can become stiffer and stronger(hypertrophy). Osteoarthritis or breakdown of cartilage due to wear andtear can also result from overloading. When cartilage breaks down, thebones rub together and cause further damage and pain. Finally, if theload rises too high, then abrupt failure of bone, cartilage and othertissues can result.

The treatment of osteoarthritis and other bone and cartilage conditionsis severely hampered when a surgeon is not able to control and prescribethe levels of joint load. Furthermore, bone healing research has shownthat some mechanical stimulation can enhance the healing response and itis likely that the optimum regime for a cartilage/bone graft orconstruct will involve different levels of load over time, e.g. during aparticular treatment schedule. Thus, there is a need for devices whichfacilitate the control of load on a joint undergoing treatment ortherapy, to thereby enable use of the joint within a healthy loadingzone.

Certain other approaches to treating osteoarthritis contemplate externaldevices such as braces or fixators which attempt to control the motionof the bones at a joint or apply cross-loads at a joint to shift loadfrom one side of the joint to the other. A number of these approacheshave had some success in alleviating pain. However, lack of patientcompliance and the inability of the devices to facilitate and supportthe natural motion and function of the diseased joint have been problemswith these external braces.

Prior approaches to treating osteoarthritis have also failed to accountfor all of the basic functions of the various structures of a joint incombination with its unique movement. In addition to addressing theloads and motions at a joint, an ultimately successful approach mustalso acknowledge the dampening and energy absorption functions of theanatomy. Prior devices designed to reduce the load transferred by thenatural joint typically incorporate relatively rigid constructs that areincompressible. Mechanical energy (E) is the action of a force (F)through a distance (s) (i.e., E=F×s). Device constructs which arerelatively rigid do not allow substantial energy storage as they do notallow substantial deformations—do not act through substantial distances.For these relatively rigid constructs, energy is transferred rather thanstored or absorbed relative to a joint. By contrast, the natural jointis a construct comprised of elements of different compliancecharacteristics such as bone, cartilage, synovial fluid, muscles,tendons, ligaments, and other tissues. These dynamic elements includerelatively compliant ones (ligaments, tendons, fluid, cartilage) whichallow for substantial energy absorption and storage, and relativelystiffer ones (bone) that allow for efficient energy transfer. Thecartilage in a joint compresses under applied force and the resultantforce displacement product represents the energy absorbed by cartilage.The fluid content of cartilage also acts to stiffen its response to loadapplied quickly and dampen its response to loads applied slowly. In thisway, cartilage acts to absorb and store, as well as to dissipate energy.

With the foregoing applications in mind, it has been found to benecessary to develop effective structures for mounting to body anatomywhich conform to body anatomy and cooperate with body anatomy to achievedesired load reduction, energy absorption, energy storage, and energytransfer. The structure should also provide a base for attachment ofcomplementary structure across articulating joints.

For these implant structures to function optimally, they should notcause a disturbance to apposing tissue in the body, nor should theirfunction be affected by anatomical tissue. Moreover, there is a need toreliably and durably transfer loads across members defining a joint.Such transfer can only be accomplished where the base structure issecurely affixed to anatomy. It has also been found desirable that abase have a smaller bone contact footprint. In this way, a less invasiveimplantable procedure can be possible, surgical time can be decreased,and larger variations in and greater members of patients can beaccommodated with the same base geometries.

Therefore, what is needed is an effective base for connecting animplantable trans-articular assembly and one which does so with areduced or minimized bone contacting surface area.

SUMMARY

Briefly, and in general terms, the disclosure is directed to bases thatare mountable to a bone and may be used for cooperation with animplantable trans-articular system. In one approach, the basesfacilitate mounting an extra-articular implantable absorber ormechanical energy absorbing system.

According to one embodiment, the bases of the energy absorbing systemare curved to match the bone surfaces of the femur and tibia and aresecured with bone screws. In one particular embodiment, the base has abone contacting surface area of less than 750 mm². In one aspect, thebase includes a total of three threaded holes for receiving lockingscrews. In a further aspect, the base includes a single hole adapted toreceive a compression screw and certain bases can further include atleast one hole sized to accept a K-wire (Kirschner wire) or Steinmannpin.

In further aspects, the base of the present disclosure contemplates theuse of locking screws with threaded heads as well as bases with threethreaded holes forming a triangular pattern. In one approach, anon-threaded hole for receiving a compression screw is configuredentirely or at least partially within an area defined by the trianglepattern. One contemplated femoral base can include three threaded holeshaving axes all three with non-parallel trajectories. Additionally, thefemoral base can include a K-wire hole having an axis which issubstantially parallel to an axis of a non-threaded opening provided fora compression screw. The tibial base can have a hole for a compressionscrew which is perpendicular to bone. Further, the position and numberof locking screw holes of the bases are selected to reduce moment forceson the bases as well as provide an anti-rotation function.

It is also contemplated that various versions of both femoral and tibialbases can be provided so that larger segments of the population can betreated. In one particular approach, three versions of femoral bases canbe provided as a kit. Such femoral bases can be characterized by theangle between the plane in which locking screws affixing the femoralbase to bone contact the bone and a line perpendicular to the sagittalplane of the patient. In this regard, angles of 40°, 45° and 50° arecontemplated.

The various tibial bases which can be provided as a kit and can include11 mm, 14 mm and 17 mm versions. Such dimensions represent the distancefrom bone to a center of rotation of a ball and socket arrangementassociated with the particular tibial base.

The femoral and tibial bases are also designed to preserve thearticulating joint and capsular structures of the knee. Accordingly,various knee procedures, including uni-compartmental and total jointreplacement, may be subsequently performed without requiring removal ofthe bases.

In one specific embodiment, the bases each include a body having aninner surface that is curved in shape to mate with a bone surface. Theinner surface contacts the bone surface and may be porous, roughened oretched to promote osteointegration. Osteointegration is a process ofbone growth onto and about an implanted device that results inintegrating the implant to the bone, thereby facilitating the transferof load and stress from the implant directly to the bone. The innersurface can be coated with an osteointegration composition. The base isalso shaped to avoid and preserve structures of the knee. Moreover, thebase is configured to locate a mounting member on the bone in order toposition a kinematic load absorber for optimal reduction of forces on ajoint. The base is a relatively rigid structure that may be made frommetal, polymer or ceramic materials including titanium, cobalt chrome,or polyetheretherketone (PEEK) or a combination thereof. In an alternateapproach, the base can be formed at least partially from flexiblematerial.

It is contemplated that the base includes a low-profile body that isgenerally elongate and includes first and second end portions. The firstend terminates in a curved manner and the second end includes structurefor mating with a mount for an absorber arrangement. The body isnon-planar such that the second end of the body is elevated as comparedto the first end of the body. In an application relating to treating aknee joint, the inner surface of the body can be curved so as to beshaped to fit to the medial surface of the femur and/or tibia onopposite sides of a knee joint. The inner surface can also be curved tomate with other surfaces such as lateral surfaces of the femur andtibia.

Other features and advantages will become apparent from the followingdetailed description, taken in conjunction with the accompanyingdrawings, which illustrate by way of example, the features of thevarious embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view, depicting an energy absorbing system attachedacross a knee joint;

FIG. 2 is a side view, depicting the system of FIG. 1 with the jointanatomy shown in a hidden format;

FIG. 3 is an enlarged side view, depicting the system of FIG. 1 removedfrom anatomy;

FIG. 4 is an enlarged side view, depicting a femoral base of the systemof FIG. 3 with a socket removed;

FIG. 5 is an enlarged side view, depicting a tibial base of the systemof FIG. 3 with a socket removed;

FIGS. 6A-6E are various angled views of the femoral base shown in FIG.4;

FIG. 7 is a perspective view, depicting three embodiments of a femoralbase;

FIGS. 8A-E are various coupled views of the tibial base shown in FIG. 5;and

FIG. 9 is a perspective view, depicting three embodiments of a tibialbase.

DETAILED DESCRIPTION

Various embodiments are disclosed which are directed to bases forattachment to body anatomy. In a preferred approach, femoral and tibialbases are provided for attachment of an extra-articular implantablemechanical energy absorbing system to the body anatomy.

In a specific embodiment, the femoral and tibial bases are shaped tomatch the medial surfaces of the femur and tibia, respectively. Thebases have a low-profile design and curved surfaces thereby minimizingthe profile of the bases when mounted to the bone surface and enablingatraumatic motion of the adjoining soft tissues over the bases. Thebases are secured to bone surfaces with one or more fastening members.

The base can be configured to be an anchor for the extra-articularimplantable absorber or mechanical energy absorbing system used toreduce forces on the knee or other joints (e.g., finger, toe, elbow,hip, ankle). The base also can be designed to distribute loads onto thebone from an extra-articular implantable absorber or mechanical energyabsorbing system while avoiding articulating joint and capsularstructures.

Various shapes of bases are contemplated and described. Moreover, it iscontemplated that various sized and similar shaped bases be madeavailable to a physician in a kit so that a proper fit to variably sizedand shaped bones can be accomplished. In that regard, it is contemplatedthat up to three or more different femoral and tibial bases can beavailable to a physician.

The bases disclosed herein are structures that are different anddistinct from bone plates. As defined by the American Academy ofOrthopedic Surgeons, bone plates are internal splints that holdfractured ends of bone together. In contrast, the bases disclosed hereinare designed to couple to and transfer loads from a absorber of animplanted extra-articular system to the bones of the joint. Furthermore,the loading conditions of a bone plate system are directly proportionalto the physiological loads of the bone it is attached to, by contrastthe loading conditions of a base is not directly proportional to thephysiological loading conditions of the bone but is directlyproportional to the loading conditions of the absorber to which it iscoupled. In various embodiments, the base is configured to transfer theload through the fastening members used to secure the base to the boneand/or one or more osteointegration areas on the base. The bases aredesigned and positioned on the bone adjacent a joint to achieve desiredkinematics of the absorber when the absorber is attached to the bases.

The approaches to the bases disclosed herein address needs of theanatomy in cyclic loading and in particular, provides an approach whichachieves extra-cortical bony in-growth under cyclic loading. In certaindisclosed applications, shear strength of about 3 MPa or more can beexpected.

Referring now to the drawings, wherein like reference numerals denotelike or corresponding parts throughout the drawings and, moreparticularly to FIGS. 1-9, there are shown various embodiments of a basethat may be fixed to a bone. The terms distal and proximal as usedherein refer to a location with respect to a center of rotation of thearticulating joint.

FIG. 1 illustrates one embodiment of an extra-articular implantablemechanical energy absorbing system 100 as implanted at a knee joint totreat the symptoms of pain and loss of knee motion resulting fromosteoarthritis of the medial knee joint. The mechanical energy absorbingsystem 100 includes femoral and tibial bases 110, 120, respectively. Anarticulated absorber 130 is connected to both the femoral and tibialbases 110, 120. As shown in FIG. 1, the knee joint is formed at thejunction of the femur 152, the tibia 154 and the fibula 156. Through theconnections provided by the bases 110, 120, the absorber assembly 130 ofthe mechanical energy absorbing system 100 can function to absorb andreduce load on the knee joint 150 defined by a femur 152 and a tibia154. According to one example, the system 100 is placed beneath the skin(not shown) and outside the joint using a minimally invasive approachand resides at the medial aspect of the knee in the subcutaneous tissue.The system 100 requires no bone, cartilage or ligament resection. Theonly bone removal being the drilling of holes for the screws whichquickly heal if screws are removed.

It is also to be recognized that the placement of the bases 110, 120 onthe bones without interfering with the articular surfaces of the jointis made such that further procedures, such as a total knee arthroplasty(TKA), unicompartmental knee arthroplasty (UKA) or other arthroplastyprocedure, can be conducted at the joint at a later date. For the laterprocedure, the bases 110, 120 can remain in place after removing theabsorber assembly 130 or both the absorber assembly and bases can beremoved. Additionally, the absorber assembly 130 can be changed out witha new absorber assembly without having to replace the bases.

The various embodiments of the bases 110, 120 describe herein may bemade from a wide range of materials. According to one embodiment, thebases are made from metals, metal alloys, or ceramics such as, but notlimited to, Titanium, stainless steel, Cobalt Chrome or combinationsthereof. Alternatively, the bases are made from thermo-plastic materialssuch as, but not limited to, high performance polyketones includingpolyetheretherketone (PEEK) or a combination of thermo-plastic and othermaterials. Various embodiments of the bases are relatively rigidstructures. Preferably, the material of the base is selected so thatbase stiffness approximates the bone stiffness adjacent the base tominimize stress shielding.

Turning now to FIG. 2, it can be appreciated that the femoral and tibialbases 110, 120 include various surfaces 170, 172 which are curved tosubstantially match the surfaces of bones to which they are affixed.Moreover, it is apparent that various affixating structures, such asscrews 180, 182, are contemplated for affixing the bases 110, 120 tobody anatomy.

With reference to FIG. 2, a femoral base 110 fixable to a medial surfaceof a femur 152 is illustrated. It is to be recognized, however, that thebase 110 can be configured to be fixed to a lateral side of the femur152 or other anatomy of the body. The femoral base 110 includes an outersurface 190 and an inner surface 170. The outer surface 190 of the basehas a low-profile and is curved to eliminate any edges or surfaces thatmay damage surrounding tissue when the base is affixed to bone. Theinner surface 170 and outer surface 190 are not coplanar and servediffering functions which the inner surface conforming to the bone shapeand the outer surface providing a smooth transition between the bone andthe absorber assembly 130. The proximal end of the outer surface 190 ofthe femoral base 110 may reside under the vastus medialis and isdesigned to allow the vastus medialis muscle to glide over the outersurface of the base.

The femoral base 110 is intended to be positioned on the femur at alocation that allows the center of knee rotation to be aligned relativeto a center of rotation of a femoral articulation, such as the ball andsocket joint 204 of the absorber assembly 130. According to oneembodiment, the base 110 is mounted to the medial epicondyle of thefemur 152 so that a mounting structure 220 (described below) connectingthe absorber to the femoral base 110 is located anterior and superior tothe center of rotation of the knee. Mounting the absorber 130 at thislocation allows the extra-articular mechanical energy absorbing system100 to reduce forces during the “stance” or weight bearing phase of gaitbetween heal strike and toe-off. Alternatively, the femoral base may bemounted at different positions on the femur to reduce forces duringdifferent phases of a person's gait.

As shown in FIG. 3, the femoral base 110 is generally elongate andincludes a first curved end 193 and a second squared mounting end 195which is raised to suspend the absorber 130 off the bone surface toavoid contact between the absorber and the knee capsule and associatedstructures of the knee joint. The body of the base 110 includes a curvedportion and the squared second end 195 is at an angle with respect tothe first end 193. It is contemplated that the absorber 130 be offsetapproximately 2-15 mm from the surface of the joint capsule. In onespecific embodiment, the entire second end 195 which is connectable withan associated socket structure 200 is offset from the capsular structureof the knee. Thus, the system 100 is extra-articular or outside of thecapsular structure of the knee. The system 100 is also trans-articularor extends across the articular structure of the joint. In oneembodiment, the second end 195 is designed to be located offsetapproximately 3 mm from the capsular structure. In another approach, theoffset is approximately 6 mm from the capsular structure. Accordingly,the base 110 allows for positioning of an extra-articular device on theknee joint while preserving the knee structures including the anteriorcruciate ligament (ACL), posterior cruciate ligament (PCL), Pes anseriustendon, and allowing future surgical procedures such as TKA or UKA.

Also shown in FIG. 2 is an embodiment of a tibial base 120 that ismountable to the medial surface of the tibia 154. As shown, the tibialbase 120 has an overall elongate shape and a curved portion end portion.An outer surface of the body 192 is curved convexly where the center ofthe body is thicker than the edges of the body. The tibial base 120 alsoincludes rounded edges in order to minimize sharp edges that mayotherwise cause damage to surrounding tissues when the base is coupledto the tibia 154. The body includes a rounded first end 196 and asquared-off second end 198 which defines an angle with respect to theelongate portion of the body. In various embodiments, the second end 198is configured to be spaced from bone as well as attach to the absorber130. The underside 172 of the body is the portion of the tibial base 120that contacts the tibia. The squared off end 198 is offset medially fromthe bone.

As best seen in FIGS. 3-5, the squared off second ends 195, 198 of thefemoral 110 and tibial 120 bases are shaped to mate with socketstructures 200, 202. In one approach, the sockets 200, 202 each includea post 210 which is press fit into a corresponding bore 220, 222 formedin the squared off ends of the bases 110, 120. The sockets 200, 202, inturn, receive ball structures forming ends of the absorber 130, as shownmost clearly in FIG. 2.

As shown most clearly in FIGS. 6B and 8B, it is contemplated that theinner surfaces 170, 172 of the bases 110, 120 can include bonecontacting surfaces 170, 172 shaped to match and directly contact thebone surface as well as curved offset surfaces 174, 176 between the bonecontacting surfaces and the squared off mounting ends 195, 198. Theseinner curved offset surfaces 174, 176 are designed to not come intocontact with bone and to provide an offset of the tibia articulation,such as the ball and socket joints 204, 206, in the medial directionfrom the joint. The inner bone contacting surfaces 170, 172 may becurved in an anterior to posterior direction as well as superior toinferior directions to conform to the shape of the typical patientfemur. According to one embodiment, the inner bone contacting surfaces170, 172 includes one or more compositions that induce osteointegrationto the cortex of long bones in the body. Additionally or alternatively,the inner bone contacting surfaces 170, 172 can be roughened or etchedto improve osteointegration. The inner bone contacting surfaces of thebases 110, 120 conform to the bone surface area. The amount of bonecontacting surface area can vary depending on the load. In theillustrated example, the amount of bone contacting surface area providedin able to support expected shear forces resulting from 60 lbs of loadas well as to counter bending moments and tensile forces on the basestending to lift the bases from the bone. The surface area of the bonecontacting surface 170 provided by the femoral and tibial bases 110, 120is significantly less than other bases due to the improved fit andimproved fixation provided by the new base shape and improved screwarrangement. For osteointegration the bone contacting surface areadesired for a base is determined based on the amount of load on theabsorber and the calculated shear strength of the interfaces between thebone and the base. For example, the surface area of the inner bonecontacting surface 170 of the femoral base 120 is less than 650 mm²,preferably less than 500 mm², for secure fixation to the femur and iscapable of carrying 60 pounds in 4 mm of compression of a kinematic loadabsorber 130. A safety factor may be built into base as larger surfacesmay be used in other embodiments. For example, a femoral base caninclude an osteointegration surface area of approximately 350 mm². Sincea limited number of base shapes and sizes are generally available to asurgeon, a perfect fit of a base to a bone is not always achieved. Witha smaller base size, an adequate fit can be achieved with a reducednumber of bases because there is less surface area to be matched withbone shape. In this way the same number of bases are also able toaccommodate a larger selection of patient anatomies.

Although the use of compression screws are described herein, the methodsand systems described can be employed without the use of a compressionscrew and may use the alternative of an instrument designed fordelivering compression while locking screws are placed.

For a tibial base 120 for secure fixation to the femur and capability ofcarrying 60 pounds in 4 mm of compression of a kinematic load absorber130, the bone contacting inner surface 172 is less than 750 mm²,preferably less than 700 mm² for secure fixation to the tibia.

In certain embodiments, the load transferred from the absorber to thebase can change over time. For example, when the base is initially fixedto the bone, the fastening members carry all the load. Over time, as thebase may become osteointegrated with the underlying bone at which timeboth the fastening members and the osteointegrated surface carry theload from the implanted system. The loading of the bases also variesthroughout motion of the joint as a function of the flexion angle andbased on patient activity.

The femoral and tibial bases 110, 120 include a plurality of openingsthat are sized to receive fastening members used to permanently securethe base to the bone. The openings define through-holes that may receivefastening members such as compression screws and/or locking screws. Theopenings may have divergent bore trajectories to further maximize thepull forces required to remove the base from the bone. Althoughdivergent bore trajectories are shown, converging trajectories may alsobe used as long as interference between the screws is avoided. Thenumber and trajectories of the openings may be varied in alternateembodiments.

As shown in FIGS. 6A-6E, the femoral base 110 includes a plurality ofopenings 230 a, 230 b, 232, 234 a, 234 b and 234 c. Openings 230 a, 230b have a diameter sized to receive standard K-wires or Steinmann pinsthat are used to temporarily locate the base 110 on the bone. Openings232 and 234 a-c are sized to receive fastening members used topermanently secure the base 110 to the bone. Opening 232 defines athrough hole for a compression screw 180, such as a cancellous bonescrews. The compression screw generates compression of bone underneaththe base. Openings 234 a-c are configured to receive locking screws 182(see FIG. 2). The locking screws 182 can include a threaded head thatengages threaded locking screw holes 234 a-c and generally do notprovide the bone compression that a compression screw does. Althoughlocking screws with threaded heads and corresponding threaded openingshave been described, other types of locking screws are also know havingheads that are locked to the base in a manner other than by threading,such as by a sliding lock on the base or an insertable locking member.In one embodiment, the locking screw openings 234 a-c are threaded andthe K-wire holes 230 and compression screw opening 232 are non-threaded.The K-wire hole 230 a has a trajectory or axis parallel to that of thecompression screw hole 232. As shown, two of the locking screw openings234 a, 234 b are located near the square mounting end 195 of the femoralbase 110 in order to receive fasteners which securely fix the base tothe bone and maximize resistance to pull-out forces and other forceswhich might tend to loosen the fasteners. A third locking screw hole 234c is spaced from the other two and closer to the first end 193 of thebase 110. The position of the three locking screw holes 234 a-c in atriangular arrangement on the base 110 functions to maximize bonequality at the fastener locations and reduce both moments and forces onthe base which might cause the base or the fasteners to loosen.

The various energy absorbing devices in the present application areshown without a protective covering or sheath but it is contemplatedthat they can be within a protective covering or sheath to protect themoving elements from impingement by surrounding tissues and to preventthe devices from damaging surrounding tissue. The bases 110, 120 may beprovided with attachment means such as holes 238 for receiving afastener to attach the sheath to the bases.

The compression screw hole 232 is positioned generally at a center ofthe femoral base 110 and at least partially within a triangle formed bythe locking screw holes 234 a-c. It is contemplated that the compressionscrew hole 232 be unthreaded and is the first hole to receive afastening structure in the form of the compression screw 180 so as topull the base 110 tightly against bone. Once the femoral base 110 is soconfigured against bone, the locking screws 182 are employed to fix thebase 110 in place. Each of the locking screw holes 234 a-c are orientedin inwardly converging, non-parallel trajectories (i.e. each of thelocking screws 182 has a trajectory converging in the direction ofinsertion with each of the other locking screws) to add strength to thefixation to bone. The parallel trajectories of the K-wire hole 230 andcompression screw hole 232 reduce or eliminate displacement of the base110 during initial fixation by the compression screw 235. The paralleltrajectory of the K-wire hole 230 also substantially eliminates theoccurrence of binding of the K-wire in the hole after screw fixation.Further, the third locking screw hole 234 c positioned near the firstend 193 of the base 110 operates to provide an anti-rotation feature.The openings 234 a-c may also have divergent bore trajectories tofurther maximize the pull forces required to remove the base from thebone. The number and trajectories of the openings may be varied inalternate embodiments.

The femoral base 110 can also be provided with a post access port 240positioned near the squared, mounting end 193 of the base 110. The postaccess port 240 is sized to receive a tool (not shown) that allows forlocking of a socket member 240 (See FIG. 4) to the base 110 by pullingthe post 210 of the socket member 240 into the base 110. It is to befurther recognized that the openings 232, 234 a-c can be countersunk toallow the fastening members to sit below the surface of the base body asshown in FIG. 2. In one specific approach, the openings 232, 234 a-c aresized to accommodate 4.0 mm screws. In other approaches, the openingsmay be sized to accommodate 3.5 mm, 4.5 mm, 5.0 mm, or 6.5 screws.

FIG. 6B illustrates a view of the inner surface 170 of the femoral base110. The inner surface bone contacting surface 170 can be roughened oretched to improve osteointegration. Alternatively, the inner surfacebone contacting surface 170 can be modified in other ways to induce bonegrowth. In one example, the inner surface bone contacting 170 may becoated with bone morphogenic protein 2 (BMP-2), hydroxyapatite (HA),titanium, cobalt chrome beads, any other osteo-generating substance or acombination of two or more coatings. According to one embodiment, atitanium plasma spray coating having a thickness of approximately 0.025in.±0.005 in. is applied to the inner bone contacting surface 170. Inanother embodiment, a HA plasma spray having a thickness ofapproximately 35 μm±10 μm is applied to facilitate osteointegration. Theportions of the inner surfaces of the base which are not in contact withthe bone including the curved offset surfaces 174 of the bases may ormay not be treated in the same manner to improve osteointegration at thebone contacting surface.

As shown in FIGS. 6C-6E, the inner surface 170 has a first radius ofcurvature at the first end 193 of the base 110 and a second radius ofcurvature at the second end 195 of the inner surface 170, where thefirst radius of curvature can differ from the second radius ofcurvature. Additionally, the inner surface 170 is generally helical inshape when moving from the first end 193 to the second end 195 of thebase 110. That is, the inner surface 170 twists when moving from the topof the inner surface to the bottom of the inner surface. The helicalnature of the inner surface 170 generally follows the shape of thedistal medial femur when moving distally (down the femur) andposteriorly (front to back). Accordingly, the curved shape of the innersurface 170 helps to reduce the overall profile of the base 110 whenaffixed to the medial surface of the femur. Additionally, the matchingcurved shape of the inner surface 28 increases the surface area in whichthe femoral base 110 contacts the femur thereby improving loaddistribution. The curved shape of the outer surface 190 softens thetransitions between the absorber 130 and the femoral base 110, betweenthe base and bone, and improves the smooth motion of skin, muscle, andother anatomy over the base.

It is contemplated that femoral base 110 can be provided in two or moreversions to accommodate patient anatomies. The two or more versions ofthe femoral base 110 form a set of bases of different shapes and/orsizes which are modular in that any one of these bases can be used withthe same absorber. In one example, three base shapes are provided anddesignated 40°, 45°, 50° bases 110 a, 110 b, 110 c (See FIG. 7). Theseangles represent the angle between a plane formed by the three pointswhere the locking screws 234 contact the bone and a line perpendicularto the saggital plane (vertical A-P plane through the joint) of thepatient. The femoral bases 110 are substantially the same size andshape, but are each rotated by 5 degrees about the center of rotation ofa ball and socket joint attached to the base (See FIGS. 1 and 2). Suchfemoral base versions allow improved kinematics by allowing the base tobe selected and placed for each particular patient in order to achieve adesired location of the center of rotation. The location of the centerof rotation of the ball and socket joint 204 at a desired locationallows improved range of motion and desired kinematics for differentpatient bone geometries. The orientation of the mounting end 195 at adesired orientation is also important to allowing desired kinematics.Placing the femoral ball and socket joint 204 at the desired locationand orientation allows controlled clearance between the bone and theabsorber 130 during full range of motion, as well as full range ofmotion of the knee without impingement of the absorber on the socket. Inone example, the desired location of the center of rotation of thefemoral ball and socket joint 204 is slightly anterior and distal to theradiographic center of rotation of the knee joint. A center of rotationof the knee joint can be approximated by locating the midpoint ofBlumensatt's line. The center of rotation of the femoral ball and socketjoint can also be arranged to be located at a desired offset distancefrom the bone. This offset distance is about 2 to 15 mm, preferablyabout 5 to 12 mm.

The implantable mechanical energy absorbing systems described hereinhave a total of 7 degrees of freedom including two universal joints eachhaving three degrees of freedom and the absorber having one degree offreedom. However, other combinations of joints may be used to form animplantable energy absorbing system, such as a system having 5 or 6degrees of freedom.

The figures have illustrated the implantable mechanical energy absorbingsystems designed for placement on the medial side of the left knee. Itis to be appreciated that a mirror image of the femoral base 110 wouldbe fixable to the medial surface of the right femur for the purposes ofunloading or reducing a load on the medial compartment of the knee. Inan alternate embodiment, the femoral and tibial bases 110, 120 and theabsorber 130 may be configured to be fixed to the lateral surfaces ofthe left or right femur and to reduce loads on the lateral compartmentof the knee. In yet another approach, implantable mechanical energyabsorbing systems can be fixed to both the lateral and medial surfacesof the left or right knee joint or of other joints.

As shown in FIGS. 8A-8E, the tibial base 120 also includes a pluralityof through holes 232, 234 a-c, 236. A non-threaded hole 232 is sized toreceive a compression screw 180 (See FIG. 2) and three threaded holes234 a-c are designed to accept locking screws 182. The compression screwhole 232 is positioned generally at a center of the tibial base 120 andat least partially within a triangle formed by the locking screw holes234 a-c. The three openings 234 a-c are oriented to provide differingtrajectories for fastening members that maximize pull out forces therebyminimizing the possibility that the tibial base 120 is separated fromthe bone. According to one embodiment, the trajectories of the lockingscrews 182 in the tibial base 120 are oriented such that the holetrajectories (axes) and corresponding locking screws are normal orapproximately normal to the shear loading forces on the base or normalto be surface of the adjacent bone. The screw trajectories are designedto minimize potential for violation of the joint space and/or posteriorjoint structures.

As with the femoral base, the openings 232, 234 a-c can be countersunkto allow the heads of fastening members to sit below the surface of thebody as shown in FIG. 2. According to one embodiment, the openings 232,234 a-c are sized to accommodate 4.0 mm diameter fastening members. Inother embodiments, the openings 232, 234 may be sized to accommodate 3.5mm, 4.5 mm, 5.0 mm or other diameter fastening members.

According to one embodiment, a femoral base 110 is implanted byselecting a base which most closely accommodates the patients bone whilelocating the femoral ball and socket articulation at a desired location,placing the base on the bone, inserting a K-wire through the opening 230a to hold the desired location, inserting the compression screw 180followed by inserting the locking screws 182. The selection of the bestfemoral and tibial bases 110, 120 can be accomplished by radiographicassessment, by providing multiple trials of the different bases formanual testing, by providing a base template which represents multiplebases, or by a combination of these or other methods.

While screws are used to fix the femoral and tibial bases 110, 120 tothe bone, those skilled in the art will appreciate that any fasteningmembers known or developed in the art may be used in addition to or asan alternative to screw fixation to accomplish desired affixation.Additional instruments and methods which are usable with the bases aredescribed in detail in U.S. Patent Application No. 61/259,052 entitled,“Positioning Systems and Methods for Implanting an Energy AbsorbingSystem,” which is incorporated herein by reference in its entirety.

The tibial base 120 may also include a plurality of holes 236 that maybe used during alignment of the base 120 on the tibia and sized toreceive structures such as a K-wire. Optionally, the base 120 mayinclude a plurality of holes, teeth or other surface features (notshown) to promote bone in-growth thereby improving base stability.

As best seen in FIGS. 8B-8E, the inner bone contacting surface 172 ofthe tibial base 120 represents the base to bone surface required tosupport expected shear forces resulting from 60 lbs of load carrying aswell as other forces on the base. The inner bone contacting surface 172can be a roughened surface for improving osteointegration. Alternativelyor additionally, the inner surface 172 can be coated to induce bonegrowth. For example, the inner surface 172 may be coated with bonemorphogenic protein 2 (BMP-2) or hydroxyapatite, titanium, cobalt chromebeads. The inner bone contacting surface 172 is a curved surface thatmatches the tibia shape and promotes good contact between the base 120and the tibia. Accordingly, the inner surface facilitates the tibialbase 120 absorbing and transferring load forces from the base to thetibia. The portions of the inner surfaces of the tibial base 120 whichare not in contact with the bone including the curved offset surfaces176 of the bases may or may not be treated in the same manner as thebone contacting surfaces 172 to improve osteointegration at the bonecontacting surface.

The tibial base 120 has a generally low-profile when mounted to thebone. The base 120 is mounted to the medial surface of the tibia inorder to preserve critical anatomy such as, but not limited to, medialcollateral ligaments. The tibial base shape is designed to remain on ananteriomedial surface of the tibia and to avoid important anatomicalstructures on the posterior aspect of the tibia.

As best seen in FIG. 2, the second end 198 of the base 120 is offsetfrom the surface of the tibia allowing the absorber to move throughout arange of motion while avoiding anatomical structures and maintaining alow profile of the base. Together the tibial and femoral bases 120, 110are configured to receive the absorber in a position where the absorberplane is substantially parallel to a line connecting the medial aspectsof the femoral and tibial condyles.

The tibial base 120 shown in the figures is configured to be fixed tothe medial surface of the left tibia. As those skilled in the art willappreciate, a mirror image of the base 120 would be fixable to themedial surface of the right tibia. Tibial bases 120 can be provided intwo or more versions to fit the different anatomy of patients. The twoor more versions of the tibial base 120 form a set of bases of differentshapes and/or sizes which are modular in that any one of these bases canbe used with the same absorber. In one example, three versions 11 mmbase 120 a, 14 mm base 120 b and 17 mm base 120 c (See FIG. 9) areprovided. These dimension identifiers represent the distance from thetibia to the center of rotation of a tibial ball and socket 206 attachedto the tibial base 120 (See also FIGS. 1 and 2). The tibial bases 120are substantially the same size and shape, but are each translated by 3mm above the bone to form the three different versions. The new baseversions allow improved kinematics by allowing bases to be placed inorder to achieve a desired location of the center of rotation. Thedesired center of rotation of the tibial ball and socket joint 206 isselected to provide controlled clearance between the absorber and theanatomical joint and to prevent impingement of the absorber on thesocket.

According to one embodiment, a tibial base 120 is implanted by firstselecting a base which most closely accommodates the patient's bone andjoint anatomy. To do this, the tibial base is positioned a set distancefrom the femoral base with the distance there between being defined bythe absorber length. Variation of this distance may occur based onabsorber compression and patient activity. Once the tibial base 120 islocated on the tibia one or more K-wires, compression screws 180 and/orlocking screws 182 are inserted in a manner similar to the method usedto secure the femoral base 110.

In one specific application, the femoral and tibial bases 110, 120 aredesigned to have a relatively small footprint which results in a lessinvasive procedure with smaller incisions needed to implant the bases.The small bases also require less periosteum elevation during surgeryresulting in a less invasive procedure. Surgical time can also beshortened by using smaller bases and associated less dissection time andinvolving fewer screws to insert. In addition to improving theimplantation procedure, the smaller bases accommodate larger variationsin patient anatomies and accommodate larger numbers of patients with thesame number of base versions. This improved fit of bases is the directresult of the fact that there is less surface area that needs to fitclosely to the bone.

The use of a single central compression screw combined with surroundinglocking screws for fixation allows fixation to be provided nearlyentirely by the screws and very little osteointegration of base to bonemay be needed. Thus, improved screw fixation is a key to fixation inplace of increase surface area.

Although the mechanical energy absorbing system 100 has been illustratedas used to reduce loading on the medial knee, it may also be used in thelateral knee as well as other joints such as the finger, hand, toe,spine, elbow, hip and ankle. Other base configurations and shapes whichmay be suitable for use in some of these applications include thosedisclosed in U.S. Patent Publication No. 2008/0275562 which isincorporated herein by reference in its entirety.

The various embodiments described above are provided by way ofillustration only and should not be construed to limit the claimedinvention. Those skilled in the art will readily recognize variousmodifications and changes that may be made to the claimed inventionwithout following the example embodiments and applications illustratedand described herein, and without departing from the true spirit andscope of the claimed invention, which is set forth in the followingclaims. In that regard, various features from certain of the disclosedembodiments can be incorporated into other of the disclosed embodimentsto provide desired structure.

1. A mechanical energy absorbing system comprising: a femoral basehaving a bone contacting surface with a surface area of less than 650mm², the femoral base having a single non-threaded opening thereinconfigured to receive a compression screw and a plurality of threadedopenings therein configured to receive a plurality of locking screwswith threaded locking heads; a tibial base having a bone contactingsurface area of less than 750 mm²; an absorber connected between thefemoral and tibial bases and configured to reduce loads born by a knee.2. The system of claim 1, wherein the femoral base has only threethreaded openings therein.
 3. The system of claim 1, wherein the tibialbase has a single non-threaded opening therein configured to receive acompression screw and a plurality of threaded openings thereinconfigured to receive a plurality of locking screws with threadedlocking heads.
 4. The system of claim 1, wherein said threaded openingsare positioned at three points of a triangle.
 5. The system of claim 4,wherein the non-threaded opening is positioned at least partially withinthe triangle.
 6. The system of claim 2, wherein the three threadedopenings each have axes which cross each other on a bone contacting sideof the femoral base.
 7. The system of claim 3, wherein said threadedopenings of the tibial base are positioned at three points of a triangleand the non-threaded opening is positioned at least partially within thetriangle.
 8. The system of claim 7, wherein two of the threaded openingsof the tibial base closest to the absorber have axes which cross eachother on a bone contacting side of the tibial base.
 9. The system ofclaim 7, wherein a threaded opening of the tibial base furthest from theabsorber has an axis which does not cross with the axes of the other twothreaded openings.
 10. A femoral base for a mechanical energy absorbingsystem comprising: a body having a bone contacting surface and anattachment site for attaching an energy absorber; a non-threaded openingformed in the body and configured to receive a compression screw; aplurality of threaded openings formed in the body and configured toreceive a plurality of locking screws with threaded locking heads; and anon-threaded K-wire opening smaller than the threaded and non-threadedopenings, the K-wire opening having an axis parallel to an axis of thenon-threaded opening.
 11. The femoral base of claim 10, wherein thenon-threaded opening is a single non-threaded opening.
 12. A method ofimplanting a femoral base for a mechanical energy absorbing systemcomprising: placing a femoral base having a bone contacting surfaceagainst the femur; inserting a K-wire through a K-wire opening in thefemoral base to hold the base in place on the femur; inserting acompression screw through a corresponding opening formed in the base;inserting a plurality of locking screws in threaded openings formed inthe base and engaging threaded heads of the locking screws with thethreaded openings, wherein the K-wire opening has an axis parallel to anaxis of the compression screw opening.
 13. The method of claim 12,wherein the compression screw opening is a non-threaded opening and isthe only non-threaded opening of the base. 14-15. (canceled)
 16. Amethod of selecting a femoral base of a mechanical energy absorbingsystem for implanting in a patient, the method comprising: providing aplurality of femoral bases having substantially the same size and shape,while being rotated with respect to one another about a center ofrotation of the mechanical energy absorbing system; and selecting one ofthe femoral bases from the plurality of femoral bases in order to locatethe center of rotation of the mechanical energy absorbing system at adesired location with respect to a center of rotation of the knee jointand at a desired offset distance from the bone.
 17. The method of claim16, wherein the desired offset distance is about 2 to 15 mm. 18.(canceled)
 19. A method of selecting a tibial base of a mechanicalenergy absorbing system for implanting in a patient, the methodcomprising: providing a plurality of tibial bases having substantiallythe same size and shape, while being translated with respect to a bonecontacting surface; and selecting one of the tibial bases from theplurality of tibial bases in order to locate the center of rotation ofthe mechanical energy absorbing system at a desired location withrespect to the bone.
 20. A mechanical energy absorbing system, thesystem comprising: a femoral base configured for implantation on amedial aspect of the femur; a tibial base configured for implantation ona medial aspect of the tibia; an absorber configured to be connected tothe femoral base and the tibial base in an position where the absorberis located in an absorber plane; and wherein the bases are configured toreceive the absorber in a position where the absorber plane issubstantially parallel to a line connecting the medial aspects of thefemoral and tibial condyles.
 21. The system of claim 20, wherein basesare configured to receive the absorber at a location offset from theline connecting the medial aspects of the femoral and tibial condyles by2-15 mm.